The present invention relates generally to dielectric filters, and, more particularly, to a dielectric filter which may be directly mounted upon a surface, such as a printed circuit board.
The design of and use of filter circuitry for filtering signals of undesired frequency components is well known. For example, elementary filter circuits performing bandpass, band reject, low pass, high pass, and combinations thereof are all well-known and are utilized to form portions of electrical circuits to pass (or reject) certain frequency portions of signals supplied to the filter circuit.
Filter circuits may be comprised of either active or passive filter components. An active filter component may be advantageously embodied in an integrated circuit, but an active filter is generally linear over only a limited dynamic range. Additionally, an active filter component exhibits proper filter characteristics over only the limited dynamic range. More conventionally, filter circuits are comprised of passive filter components, such as combinations of resistors, capacitors, and inductors. The resistive, capacitive, and inductive component values of the passive filter components, and their electrical connection therebetween, define a resonant frequency of the filter circuit. The passive filter components may be connected in manners, and may be of resistive, capacitive, and inductive values, to form a filter circuit, such as any of the above-listed filter circuits, for passing (or rejecting) signal portions of any desired frequencies.
For example, filter circuits forming a portion of an electrical circuit may be positioned in a series connection with the electrical circuit. As mentioned hereinabove, the values of the filter components and their electrical connection therebetween defines a resonant frequency. When the filter circuit is connected in a series connection with other portions of the electrical circuit, signal portions of a signal supplied to the filter circuit within the resonant frequency defined by the filter circuit are passed by the filter circuit. By appropriate selection of the component values of the components of the filter circuit as well as their electrical connection therebetween, a passband filter circuit, for example, may be constructed to pass signal portions of any desired frequencies.
Filter circuits forming a portion of an electrical circuit may also be positioned in a shunt connection with other portions of the electrical circuit (i.e., be positioned to extend between the electrical circuit and a ground plane). As mentioned hereinabove, the values of the filter component and their electrical connection therebetween defines a resonant frequency. When the filter circuit is connected in a shunt connection with the electrical circuit, signal portions of a signal supplied to the filter circuit within the resonant frequency defined by the filter circuit are shunted to ground by the filter circuit. By appropriate selection of component values of the components of the filter circuit, as well as their electrical connection therebetween, a band reject filter circuit, for example, may be constructed to reject signal portions of any desired frequencies.
An electrical circuit may also contain combinations of both series and shunt filter circuits to perform circuit functions as desired.
Electrical circuits which utilize filter circuitry to form a portion thereof include radio frequency receiver circuits. Filter circuitry is utilized in a radio frequency receiver circuit to tune the receiver, and, additionally, to filter intermodulation spurs generated during down conversion of a received signal. While an ideal receiver reproduces only the signal transmitted thereto, an actual, nonideal receiver, through a process of mixing occurring during down conversion of a received signal, produces intermodulation distortion. Associated with intermodulation distortion are undesired spurious signals generated during down conversion of a signal received by a nonideal receiver. Such undesired, spurious signals are the intermodulation spurs referred to hereinabove. Filter circuits may, of course, be utilized by the receiver circuits to perform other filter functions.
Ceramic, and other dielectric filters, are oftentimes utilized to form passive filter circuits. Such filter circuits, frequently referred to as ceramic block filters due to the geometry of most of such filters, are advantageously utilized to form a passive filter circuit as a single ceramic block may be formed to comprise a filter. The ceramic block filter may be connected in series, or in shunt, to perform a filter function as desired. Typically, the filter is formed in the shape of a block, and one or more holes are drilled or otherwise created to extend into the block. Portions of the sidewalls defining the hole are coated with an electrically conductive material; additionally, surface portions of the ceramic block are also covered with an electrically conductive material. The holes drilled to extend into, or through, the dielectric block, once coated with the electrically conductive material, form resonators of characteristics defined by the length and surface area of the sidewalls which define the resonator. While the resonators may extend in any direction, typically, the resonators are formed to extend from the front portion to the rear portion or to extend between opposing side portions of the block.
U.S. Pat. Nos. 4,431,977 and 4,742,562 disclose examples of such ceramic block filters. The ceramic block filters disclosed in the aforementioned two patents are coupled to an electrical circuit by integral, conventional coupling connectors, and integral coaxial coupling cables, respectively. U.S. Pat. No. 4,673,902 discloses a ceramic block filter which is coupled directly to an electrical circuit by a direct solder connection, thereby obviating the need for coaxial cable to connect the ceramic block filter to the circuit. Additionally, the ceramic block filter is susceptible to movement when the solder becomes liquid during the solder reflow operations. Such movement can result in inadequate electrical connection of the filter to the electrical circuit and other misalignment problems.
What is needed, therefore, is a ceramic block filter construction which, once positioned upon the circuit board, does not become misaligned during solder reflow operations.
Additionally, U.S. Pat. No. 4,703,291 discloses a ceramic block filter in which wires interconnect the resonators of the ceramic block filter and the electrical circuit. U.S. Pat. No. 4,716,391 discloses a ceramic block filter in which wires interconnect the resonators formed in the ceramic block filter and input/output pads.
Because many electrical devices are packaged in ever-smaller housings, the electrical circuits comprising portions of the electrical devices must be miniaturized in order to fit within the ever-smaller housings. For example, portable transceivers, such as portable, cellular phones, are increasingly miniaturized to permit the transceivers to be of ever-smaller dimensions. Because of the block-like shape of a ceramic block filter, when the ceramic block filter forms a portion of an electrical circuit disposed upon a circuit board, a minimum heightwise spacing is required above the circuit board to permit positioning of the ceramic block filter thereupon. This heightwise spacing required to permit positioning of the ceramic block filter upon a circuit board may limit the miniaturization permitted of an electrical device. Therefore, attempts have been made to most efficiently position a ceramic block filter upon a circuit board to permit continued miniaturization of an electrical device. For instance, U.S. patent application Ser. No. 455,062 filed on Dec. 22, 1989, discloses a surface mount filter which is positioned to extend through an opening of dimensions to permit insertion of the ceramic block filter therethrough. A bracket is positioned about the ceramic block filter to affix the filter to the circuit board. The necessity of a bracket, however, requires an extra production step during production of the electrical circuit, thereby increasing production costs of the electrical circuit.
What is needed, therefore, is a filter mountable upon a support surface, such as a circuit board, which minimizes the heightwise requirements of the filter when mounted thereupon, and which, at the same time, may be mounted in position in a manner which minimizes production costs.